Any valid style sheet is allowed for your use with the Upload Custom Header/Footer/Images option. We have defined a couple style classes that the shopping cart will refer to in order to allow you to further customize the look of your cart

productHeading - Allows you to define a style for the header at the top of your product list

productLineItem - Allows you to define a style to be used for each product line item listed in the cart.

productLineItemDescription - Allows you to define a style to be used for each product description listed in the cart.

Total - Allows you to adjust the look and feel for the grand total

These styles are used as shown here:
<tr align=center valign=middle class="productHeading">

And you can modify their display by creating a standard style definition in your custom.html file like so:
<style>
TR.productHeading { font-size: 15px; }
</style>